ARTICLE pubs.acs.org/JPCC
Pillared Covalent Organic Frameworks with Balanced Volumetric and Gravimetric Hydrogen Uptake Daejin Kim,†,‡ Dong Hyun Jung,† Kyung-Hyun Kim,† Hyein Guk,† Sang Soo Han,§ Kihang Choi,*,‡ and Seung-Hoon Choi*,† †
Insilicotech Co., Ltd., C-602, Korea Bio Park, 694-1, Sampyeong-dong Bundang-gu, Seongnam 463-400, Korea Department of Chemistry, Korea University, 1, Anam-dong 5-Ga, Seongbuk-Gu, Seoul 136-701, Korea § Korea Research Institute of Standards and Science, 209 Gajeong-Ro, Yuseong-Gu, Daejeon 305-340, Korea ‡
bS Supporting Information ABSTRACT: On the basis of our modeling of pillared covalent organic frameworks (PCOFs) with pyridine molecules inserted between the COF-1 layers, we propose that the surface area and free volume of storage materials should be balanced to increase the gravimetric and volumetric hydrogen uptake capacities. Density functional theory and grand canonical Monte Carlo simulations show that these PCOFs have significantly improved gravimetric and volumetric hydrogen storage capacities of 8.810.0 wt % and 58.761.7 g L1, respectively.
’ INTRODUCTION As potential candidate materials for hydrogen storage, MOFs (metalorganic frameworks) and COFs (covalent-organic frameworks) are appealing because they can take up and release hydrogen reversibly with fast kinetics.18 Because they work effectively only at low temperature due to their low hydrogenbinding energies, several strategies such as metal or metal ion doping have been suggested by modeling studies to increase the binding energy up to about 4 to 5 kcal mol1.8,9 However, it is very challenging to synthesize these materials because of the difficulties in dispersing the reactive metal centers on the surface of sorbents, although some MOFs and COFs show impressive material-based gravimetric total capacitymore than 10 wt % with large surface areasat low temperature. The porous materials with ultrahigh surface areas have low density, which limits their volumetric capacity to ∼40 g L1.9 When the materials are included in a storage system (e.g., a high-pressure cylinder), their volumetric capacity may be reduced greatly depending on the system design. This situation requires new materials that have both large surface areas and higher hydrogen adsorption enthalpies simultaneously.10,11 This issue has not been seriously considered and henceforth remains as another challenge. Compared with MOFs, organic networks such as PAF (porous aromatic framework), PPN (porous polymer network), and COFs may have an advantage in increasing gravimetric hydrogen storage capacity because their frameworks are light. In addition, they can have very large BET surface areas (SBET), as seen in PAF-1 (5640 m2 g1).12 However, whereas COFs are crystalline, both PAF and PPN are amorphous. This makes it difficult to simulate their porous properties by theoretical methods. In this regard, COFs are good for theoretical studies to assess their r 2011 American Chemical Society
hydrogen storage behaviors by various calculation methods. Presently, COF-102 has the record value of hydrogen storage amount: 72 mg g1 (6.7 wt %, excess) at 35 bar and 77 K.9 Indeed, this result is due to their large SBET of 3620 m2 g1. However, COF-102 has a low density of 0.43 g cm3, which is reflected in the relatively low volumetric capacity of 40 g L1. It is trivial to ask what the changes in volumetric capacity will be if a COF has a smaller surface area. For example, 2D COF-1 has a SBET = 750 m2 g1 and an excess hydrogen uptake of 14.8 mg g1 (1.46 wt %, saturated uptake at 77 K). However, its volumetric capacity of 60 g L1 is much larger than that of a highly porous 3D COF-102. We have noticed that a boron atom acts as a Lewis acid; therefore, a Lewis base such as pyridine can be attached to it to make the coordination geometry tetrahedral.13 Beckmann and coworkers explained the stabilization of triphenylboroxine ring (PhBO)3 by the addition of amine in terms of ring strain:14 the intrinsically existing ring strain of (PhBO)3 can be partially released by changing the bonding geometry of one of the three boron atoms into the amine-bonded tetrahedral structure. There are several examples of molecular structures found in the Cambridge Structural Database with a boroxine ring bonded to a pyridyl group.1419 Therefore, if pyridine molecules would be bound to boron atoms of 2D COF-1, then more stabilized structure could be obtained. On the basis of this idea, we modeled corrugated forms of COF-1 by attaching pyridine molecules to boroxine rings. This modification in 2D COF-1 Received: August 22, 2011 Revised: November 23, 2011 Published: November 30, 2011 1479
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Scheme 1. Insertion of Pyridine into COF-1 As an Interlayer Spacer
Figure 1. Molecular structures of PCOF-1s. (a) Cluster model of the building unit. Stacking structures of (b) e-PCOF-1 and (c) e’-PCOF-1 with the eclipsed form as well as (d) s-PCOF-1 with the staggered form. Two adjacent layers of PCOF-1s are depicted using space-filling models with different colors. Carbon, boron, oxygen, and nitrogen atoms in upper layer are colored in gray, orange, red, and blue, respectively. All atoms in lower layer are in dark green. Hydrogen atoms are deleted for clarity.
frameworks results in semi-3D structures with large surface area, for which we have done modeling and hydrogen storage assessment using multiscale calculations. Herein we propose a pillared COF (PCOF) that has very large hydrogen uptake capacities in both gravimetric and volumetric aspects.
’ COMPUTATIONAL SECTION An in silico method for making new COFs involves replacement of the organic linker in a COF with an expanded linker or a derivative. This straightforward approach follows the isoreticular expansion strategy already demonstrated by experiments and therefore can be regarded as reasonable and safe.2 Although this method is very useful for screening purposes, it is limited only to known framework types. In addition, the expansion of the linkers does not guarantee the realization of porosity because a large void space permits framework interpenetration or causes the thin framework to collapse in vacuum. This consideration led us to adopt a new approach, of inserting pillar molecules2023 into 2D COF-1 layers to expose buried framework surfaces to pores (Scheme 1). To design and evaluate pillared covalent organic frameworks (PCOFs), we performed density functional theory (DFT) calculations with dispersion corrections (DFT-D)24 using MS CASTEP 5.5.25 From the DFT-D results, we calculated the reaction energies of Scheme 1 for each PCOF model to estimate synthetic possibility of models. On the basis of the stable structure of the pyridineboroxine ring complex (Figure 1a),13 we constructed periodic models of pillared COF-1 (PCOF-1) and found energetically stable structures, for which the uptake of hydrogen molecules was predicted by grand canonical Monte Carlo (GCMC) simulations using MS Sorption.26 The forcefield parameters used in the GCMC simulations to describe the interactions between H2 and PCOF-1s were derived from ab initio calculations on the level of second-order Møller Plesset (MP2) and DFT with the M06-2X functional,2729 using NWChem 5.1.30 We used interactions between the pyridineboroxine ring complex (Figure 1a) and H2 from DFT (M06-2X functional) as reference data for the validation of fitted forced field parameters. Interaction between the H2 and the pyridineboroxine ring complex was calculated for a series of configurations along three 1480
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Table 1. Calculated Properties of COF-1 and PCOF-1 Structures material
RE/kcal mol1a
COF-1
SE/kcal mol1b
dpyridine/Åc
dlayer/Åd
F/g cm3e
S/m2
V/cm3
H2
H2
Qst/kcal
g1f
g1g
uptake/wt %
uptake/g L1
mol1h
26.03
N/A
3.21
1.02
937
0.11
3.6
37.8
1.28
e-PCOF-1
0
22.04 (1.41)
5.86
7.80
0.61
2780
0.33
8.8
58.7
2.37
e-PCOF-1
2.39
19.64 (0.26)
3.80
8.60
0.55
3595
0.44
10.0
60.9
1.08
s-PCOF-1
3.90
18.22 (1.32)
6.72
7.70
0.60
3099
0.30
9.3
61.7
1.85
a
Relative energy per [(Ph0.5BO)3 3 Py]2. b Interlayer separation energy per [(Ph0.5BO)3]2 for COF-1 and [(Ph0.5BO)3 3 Py]2 for PCOF-1s. The value in the parentheses is SE of COF-1 when the interlayer spacing is expanded to that of PCOF-1. c Distance between adjacent pyridine rings. d Interlayer spacing. e Density. f Specific surface area. g Specific free volume. h Hydrogen adsorption enthalpy at 0.001 bar, 77 K. All H2 uptake values are calculated at 77 K and 100 bar. The specific surface area (solvent-accessible surface area) and specific free volume are calculated with a probe radius of 1.8 Å.32
Figure 3. Calculated adsorption enthalpy (Qst) of hydrogen at 77 K.
carried out RI-MP2/cc-pVDZ-fit2-1 calculations for the interaction of H2 with boroxine, benzene, and pyridine molecules. After comparison of these results with DFT level calculations, M062X/6-31G** proved suitable for calculation without loss of accuracy (Figure S1 in the Supporting Information). Details of the calculation methods and the obtained LJ parameters are described in the Supporting Information.
Figure 2. Simulated hydrogen adsorption isotherms for COF-1 and PCOF-1 structures at 77 K (gravimetric uptake in wt % on the top; volumetric uptake in g L1 on the bottom). The experimental isotherm of COF-1 shows data reported by Furukawa et al.9
directions (Figure S2 of the Supporting Information). The DFT calculation results were then fitted to obtain Lennard-Jones (LJ) 126 pair potential parameters (eq 1) between each atom of the PCOF with H 2 !12 !6 3 R R 0 0 5 ð1Þ 2 Uij ðRij Þ ¼ D0 4 Rij Rij where the parameter D0 is the well depth and R0 is the equilibrium bond distance. To verify the accuracy of DFT calculations for the weak interaction between the sorbent and a hydrogen molecule, we
’ RESULTS AND DISCUSSION COF-1 is composed of layers that are formed by selfcondensation of boronic acids, resulting in hexagonal porous frameworks. These layers are stacked in alternation along the crystallographic c axis, where each second layer is shifted so that the boroxine rings of one layer lie on top of the hexagonal pores of adjacent layers. Using this prototype structure, pyridines have been attached to one of the boron atoms in the boroxine ring in a symmetrical fashion to give s-PCOF-1 (staggered PCOF-1). In addition, two structural isomers are found when the neighboring layers are moved to eclipse each another. The two eclipsed structures are very similar, but the e’-PCOF-1 layers are shifted slightly from each other so that the adjacent pyridine rings are closer in e’-PCOF-1 than in e-PCOF-1. These PCOF-1 models were optimized using the DFT-D method (Figure 1). In all of the resulting structures, the local geometry of the pyridine-bound boroxine ring agrees well with the cluster model of the building unit.13 Upon the introduction of pyridine, the flat layer in COF-1 becomes corrugated in the new models. Reaction energies of PCOF-1s by DFT-D calculation are 9.99 to 13.89 kcal mol1 for the addition of one pyridine molecule, which demonstrates 1481
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Figure 4. Contour map of interaction energy at 0.1 bar and 77 K between molecular hydrogen and (a) COF-1, (b) e-PCOF-1, (c) e’-PCOF-1, and (d) s-PCOF-1. The map shows the cross section at the center of the benzene ring (a) or at the center of the pyridine ring (bd). ‘P’ indicates a pyridine site. (See the text.) The scale bar in panel a shows the interaction energy in kilocalories per mole.
the thermodynamic feasibility of the pyridine addition reaction. This result is consistent with the theoretical study showing the addition of pyridine molecule to flat boroxine ring results in a more strain-relieved structure.14 As the introduction of pyridine increases the distance between COF layers from 3.21 Å to as much as 8.60 Å (Table 1), it should be checked whether the lattice stability can be maintained in PCOF-1s. Interlayer separation energy, the energy required to separate layers, can be a good measure for the lattice stability of PCOF-1s and is calculated by subtracting the energy of single layer from the energy of layered structure. For the normalization, obtained interlayer separation energies are transformed as those per unit cell (i.e., [(Ph0.5BO)3]2 for COF-1 and [(Ph0.5BO)3 3 Py]2 for PCOF-1s). The calculated interlayer separation energies of PCOF-1s are from 18.22 to 22.04 kcal mol1; although these values are less than that of COF-1 (26.03 kcal mol1), they are still very large compared with those for COF-1s with the same layer spacing as PCOF-1s (Table 1). Therefore, PCOF-1s are expected to have relatively stable lattice structures in spite of the increased interlayer spacing. The accessible surface areas are calculated to be ∼3000 m2 g1 when the radius of the probe is 1.8 Å.32 These values are more than three or four times the surface area of COF-1. Hence, it can be expected that the hydrogen storage capacity is higher for PCOF-1s than for COF-1. Indeed, both gravimetric and volumetric hydrogen uptake values are significantly improved by ca. 250 and 150%, respectively (Table 1, Figure 2). Although hydrogen adsorption enthalpy of COF-1 is 1.28 to 1.39 kcal mol1 (Figure 3) comparable to the experimental value of 1.48 kcal mol1,9 the agreement between the simulated and experimental isotherms is not good, as shown in Figure 2. This disagreement can be attributed to two reasons. First, the discrepancy may be caused by incompleteness of the crystal structure used in the experiment. The structure of COF-1 used in this work is based on the report of Lukose et al.31 and reoptimized using the DFT-D method. Experimental isotherms are from the report by Furukawa and Yaghi,9 who confirmed such a disorder by observing changes in the powder X-ray diffraction (PXRD) spectra taken before and after an
adsorptiondesorption cycle. Lukose et al. claimed that all reported 2D COF geometries should be re-examined carefully by experiment.31 Furthermore, real materials have nanoporous defects and inaccessible pores due to solvent molecules left in the pore or fluid cluster formations that may stop arriving molecules from entering the pore, whereas the GCMC simulations were carried out in perfect crystals. The other reason is that hydrogen adsorption at low temperature such as 77 K is influenced by quantum effect; without the quantum effect, the simulations could overestimate the adsorption ability of the host materials.6 In this study, we ignored the quantum effect, and this could make discrepancies between the calculated isotherm and the experimental one of COF-1. To obtain the more accurate isotherm, the quantum effect should be implemented in GCMC simulations. However, as the chemical compositions are not too much different between COF-1 and PCOF-1s and hence the magnitude of zeropoint energy calculated as the quantum effect would be similar, the trends of these capacities will be consistent regardless of inclusion of quantum effect.33,34 Although all PCOF-1s have higher volumetric and gravimetric capacities than COF-1, some noteworthy differences are observed in their adsorption behaviors. Although e’-PCOF-1 exhibits the slowest initial increase in hydrogen adsorption, it has the highest gravimetric uptake among the PCOF-1s at 100 bar. As will be discussed below, the small hydrogen uptake at low pressure and low hydrogen adsorption enthalpy10 (Figure 3) can be explained by its small pyridine sites (Figures 1 and 4). In contrast, e-PCOF-1 and s-PCOF-1 have pyridine sites large enough for hydrogen uptake such that the adsorption enthalpy is high in the initial stage. As these high binding energy sites become saturated by the successive loading of hydrogen molecules, the adsorption enthalpy decreases (Figure 3). The pillar molecules not only increase the interlayer spacing but also can create additional hydrogen adsorption sites. An upward bound pyridine molecule of the lower layer meets face to face with a downward bound pyridine molecule of the upper layer, and the distances between the adjacent pyridine rings of the PCOF-1 structures are calculated to be 3.806.72 Å. According 1482
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Figure 4) turns out to be the most favorable hydrogen binding site provided by e-PCOF-1 and s-PCOF-1. As our previous simulation studies on interpenetrating MOFs demonstrated,35 the van der Waals potentials of confining walls in close proximity can overlap with one another, increasing the interaction energy between the adsorbent and hydrogen molecules. However, if the spacing between the pyridine rings is too small, then the contribution of the pyridine sites to total hydrogen uptake becomes limited. Given that e’-PCOF-1 has the smallest distance between the pyridine rings (3.80 Å), its pyridine sites are not available for hydrogen uptake (Figure 4c). Hence, e’-PCOF-1 is predicted to have the lowest adsorption enthalpy among the PCOF-1s. The population analysis of interaction energies clearly shows that e’-PCOF-1 has only one type of binding site (Figure 5). Whereas two main distribution peaks are found for e-PCOF-1 and s-PCOF-1, only one peak is observed for e’-PCOF-1. This peak is shifted to the high binding energy region with increasing pressure because as hydrogen loading increases, the interaction between the adsorbed hydrogen molecules themselves becomes stronger. The density of hydrogen absorbed in e’-PCOF-1 at 77 K and 100 atm is calculated to be 0.090 g cm3, which is comparable to the density of liquid hydrogen at the normal boiling point (0.071 g cm3). Similar results are commonly observed for MOFs and COFs. Not only the gravimetric capacity, but also the volumetric capacity of storage materials must be improved to be adopted in vehicles. In this work, the volumetric hydrogen uptake capacity is also significantly higher for PCOF-1s (∼60 g L1 at 77 K) than for COF-1. Although the gravimetric hydrogen uptakes of the PCOF-1s cannot match the best records of MOFs (14.1 wt % for NU-100 and 14.3 wt % for MOF-210 at 77 K), the volumetric hydrogen uptakes of PCOF-1s are certainly larger than those of the MOFs with high gravimetric capacity (48 g L1 for NU-100, and 42 g L1 for MOF-210 at 77 K).36,37 This increase in gravimetric and volumetric capacities is apparently related to the effective utilization of free volume, that is, the balance between density and surface area. When a COF or MOF has a large surface area and a very low density, a large portion of its pore space is ineffective for hydrogen storage, resulting in high gravimetric uptake but low volumetric uptake. In the case of PCOF-1s, although they are denser than highly porous MOFs and COFs, PCOF-1s still have significantly large surface areas that are formed not by the small number of large pores but by the large number of small pores. In addition, the hydrogen adsorption enthalpy of the PCOF-1s (∼1.8 kcal mol1) is close to the highest values of MOFs and COFs.9 On the basis of the calculated results, it is concluded that the incorporation of pyridine pillars into COF-1 adjusts the host frameworks to possess both increased surface areas and hydrogen binding energies. It is envisioned that other molecules that can act as Lewis bases might also serve as pillar molecules controlling the interlayer spacing of other types of 2D COFs.
Figure 5. Population analysis of the interaction energies of COF-1 and PCOF-1 structures at 77 K. P(E) represents the normalized population of the interaction energy.
to interaction energy analyses using GCMC simulations, the spacing between the pyridine rings (denoted as pyridine site ‘P’ in
’ CONCLUSIONS In conclusion, we have investigated the hydrogen storage properties of pillared COFs using GCMC simulations. Although 3D COFs show higher gravimetric capacities, volumetric capacities and hydrogen adsorption energies are relatively small: excess hydrogen capacity of COF-102 is 72 mg g1 (6.7 wt %) and 40 g L1 and hydrogen adsorption enthalpy is 0.9 kcal mol1.9 On the other side, COF-1, one of the 2D COFs, has higher hydrogen adsorption 1483
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’ ASSOCIATED CONTENT
bS
Supporting Information. Computational details and structural parameters for the modeling and force field parametrization of PCOF-1s and tables of the total H2 adsorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Authors
*Phone: +82-31-628-0309; Fax: + 82-31-628-0333; E-mail: shchoi@ insilicotech.co.kr (S.-H.C.). Phone: +82-2-3290-3141; Fax: + 82-2-3290-3121; E-mail:
[email protected] (K.C.).
’ ACKNOWLEDGMENT This research was performed for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Programs funded by the Ministry of Education, Science, and Technology of Korea. We thank Professor Jaheon Kim (Soongsil University) for his helpful comments and also Accelrys Korea for their support of the modeling software. ’ REFERENCES (1) C^ote, A. P.; Benin, A. I.; Ockwig, N. W.; O’Keeffe, M.; Matzger, A. J.; Yaghi, O. M. Science 2005, 310, 1166–1170. (2) C^ote, A. P.; El-Kaderi, H. M.; Furukawa, H.; Hunt, J. R.; Yaghi, O. M. J. Am. Chem. Soc. 2007, 129, 12914–12915. (3) El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Cortes, J. L.; C^ ote, A. P.; Taylor, R. E.; O’Keeffe, M.; Yaghi, O. M. Science 2007, 316, 268–272. (4) Li, Y. W.; Yang, R. T. AIChE J. 2008, 54, 269–279. (5) Han, S. S.; Furukawa, H.; Yaghi, O. M.; Goddard, W. A., III. J. Am. Chem. Soc. 2008, 130, 11580–11581. (6) Garberoglio, G. Langmuir 2007, 23, 12154–12158. (7) Klontzas, E.; Tylianakis, E.; Froudakis, G. E. J. Phys. Chem. C 2008, 112, 9095–9098. (8) Choi, Y. J.; Lee, J. W.; Choi, J. H.; Kang, J. K. Appl. Phys. Lett. 2008, 92, 173102. (9) Furukawa, H; Yaghi, O. M. J. Am. Chem. Soc. 2009, 131, 8875– 8883. (10) Frost, H.; D€uren, T.; Snurr, R. Q. J. Phys. Chem. B 2006, 110, 9565–9570. (11) Bae, Y. S.; Snurr, R. Q. Microporous Mesoporous Mater. 2010, 132, 300–303. (12) Ben, T.; Ren, H.; Ma, S.; Cao, D.; Lan, J.; Jing, X.; Wang, W.; Xu, J.; Deng, F.; Simmons, J. M.; Qiu, S.; Zhu, G. Angew. Chem., Int. Ed. 2009, 48, 9457–9460. (13) Kim, D.; Jung, D. H.; Choi, S. -H.; Kim, J.; Choi, K. J. Korean Phys. Soc. 2008, 52, 1255–1258. (14) Beckmann, J.; Dakternieks, D.; Duthie, A.; Lim, A. E. K.; Tiekink, E. R.T. J. Organomet. Chem. 2001, 633, 149–156. (15) Beckett, M. A.; Strickland, G. C.; Varma, K. S.; Hibbs, D. E.; Hursthouse, M. B.; Malik, K. M. A. J. Organomet. Chem. 1997, 535, 33–41.
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